In some networks, a network node of the network may have a local oscillator (LO) that is used for timing purposes at the network node. To reduce the cost of the network node, the LO may not be as accurate as a master timing reference source that provides a timing signal to the network.
When an external timing reference source is used to discipline the LO, if for some reason the external timing reference is unable to be used to discipline the local oscillator, for example in the case where a GPS signal is used as the external timing reference source and the GPS lock is lost, or if a network malfunction means that a network timing reference is unavailable, then performance of the network node and any other nodes that may rely on the network node's local timing signal may be impaired. Once the external timing reference is unavailable then human intervention may be required to correct or reset the timing of the network node. However, human intervention can be time consuming and costly to implement.
When the external timing reference is unavailable, it is still desirable to maintain the operation of the local oscillator such that over a duration of time an error in the local oscillator output is limited to a particular value.
In the Worldwide Interoperability for Microwave Access (WiMAX) standard for example, the particular value of time error is 25 μs over a duration of 24 hours. In 4G long term evolution (LTE) systems, the duration is even more stringent, namely 10 μs over 24 hours.